Power management (PM) and radio frequency (RF) amplification are critical device processes performed in the operation of modern mobile computing platforms, such as smartphones, tablets, and laptop/notebooks. Integrated circuits (IC) contained in System-on-Chip products and designed to perform these operations, such as power management integrated circuits (PMIC) and radio frequency integrated circuits (RFIC), require transistors that can withstand high voltages and electric fields. Typical voltages encountered by PMICs and RFICs that perform high-voltage switching for DC-to-DC conversion in the output filter as well as in the drive circuitries, for example, can be as much as 3.7 V as outputted by ordinary lithium batteries. Using silicon transistors to perform at these high voltages, however, proves difficult due to the low band gap of silicon (i.e., 1.12 eV). For instance, in order for a silicon transistor in a silicon-based PMIC to withstand voltages of 3.7 V, the transistor size will need to be in the dimension of tens of millimeters. In an alternative solution, silicon transistors in a PMIC can be formed in series. However, such configurations have significant power losses and high resistances, which lead to short battery life and cooling issues. As a result, current solutions utilize alternative semiconductor materials with wider band gaps. One such material is gallium nitride (GaN).
GaN is a wide band gap (i.e., 3.4 eV) semiconductor material that has been widely explored for its beneficial properties relating to micro-electronic devices including, but not limited to, transistors, light emitting diodes (LED), and high-power integrated circuits. GaN has a wurtzite crystalline structure with a lattice constant that is smaller than the lattice constant of silicon, and has an electron mobility similar to that of silicon, which is approximately 1300 cm2 (v·s)−1.
Currently, GaN is grown heteroepitaxially on non-GaN substrates by brute force (e.g., direct growth of epitaxial GaN on non-GaN substrates). Brute force growth of GaN on non-native substrates results in substantial lattice mismatch between the substrate and the epitaxial layer caused by the difference in their lattice structures and/or lattice constants. Lattice mismatch between a non-GaN substrate and a GaN epitaxial layer causes threading dislocation defects to propagate in all directions from the interface between the GaN epitaxial layer and non-GaN substrate.
In an effort to decrease the amount of these defects, conventional solutions grow a thick buffer layer (e.g., greater than 1 μm) of GaN on a non-native substrate (e.g., silicon, sapphire, or silicon carbide) in hopes that a number of threading dislocations will cease to occur somewhere in the middle of growth. Even with several microns of buffer GaN growth, however, the defect density of resulting GaN cannot achieve a defect density less than 2E7 cm−2. Furthermore, the buffer layer creates a large height difference between GaN transistors formed on top of the buffer layer and other transistors formed on the silicon substrate, such as complementary metal oxide semiconductors (CMOS). As a result, this height difference precludes direct heterogeneous integration of GaN transistors on silicon substrates for co-integration with silicon CMOS transistors on the same substrate plane.